The use of magnetic field measurements in prior art subterranean surveying techniques for determining the direction of the earth's magnetic field at a particular point is well known. Techniques are also well known for using magnetic field measurements to locate subterranean magnetic structures, such as a nearby cased borehole. These techniques are often used, for example, in well twinning applications in which one well (the twin well) is drilled in close proximity and often substantially parallel to another well (commonly referred to as a target well).
The magnetic techniques used to sense a target well may generally be divided into two main groups; (i) active ranging and (ii) passive ranging. In active ranging, the local subterranean environment is provided with an external magnetic field, for example, via a strong electromagnetic source in the target well. The properties of the external field are assumed to vary in a known manner with distance and direction from the source and thus in some applications may be used to determine the location of the target well. In contrast to active ranging, passive ranging techniques utilize a preexisting magnetic field emanating from magnetized components within the target borehole. In particular, conventional passive ranging techniques generally take advantage of remanent magnetization in the target well casing string. Such remanent magnetization is typically residual in the casing string because of magnetic particle inspection techniques that are commonly utilized to inspect the threaded ends of individual casing tubulars.
In co-pending U.S. patent application Ser. No. 11/301,762 to McElhinney, a technique is disclosed in which a predetermined magnetic pattern is deliberately imparted to a plurality of casing tubulars. These tubulars, thus magnetized, are coupled together and lowered into a target well to form a magnetized section of casing string typically including a plurality of longitudinally spaced pairs of opposing magnetic poles. Passive ranging measurements of the magnetic field may then be advantageously utilized to survey and guide drilling of a twin well relative to the target well. This well twinning technique may be used, for example, in steam assisted gravity drainage (SAGD) applications in which horizontal twin wells are drilled to recover heavy oil from tar sands.
McElhinney discloses the use of, for example, a single magnetizing coil to impart the predetermined magnetic pattern to each of the casing tubulars. As shown on FIG. 1A, a hand-held magnetizing coil 80 having a central opening (not shown) is deployed about exemplary tubular 60. A DC current is passed through the windings in the coil 80 (the current traveling circumferentially about the tubular), which imparts a substantially permanent, strong, longitudinal magnetization to the tubular 60 in the vicinity of the coil 80. After some period of time (e.g., 5 to 15 seconds) the current is interrupted and the coil 80 moved longitudinally to another portion of the tubular 60 where the process is repeated, thereby longitudinally magnetizing another region of the tubular 60. To impart a pair of opposing magnetic poles 65 (FIG. 1B), McElhinney discloses reversing the direction of the current about coil 80 or alternatively redeploying the coil 80 about the tubular 60 such that the electric current flows in the opposite circumferential direction, thereby imparting a longitudinal magnetization having the opposite polarity.
FIG. 1B depicts an exemplary tubular 60 magnetized as described above with respect to FIG. 1A. As shown, tubular 60 includes a plurality of discrete magnetized zones 62 (typically three or more). Each magnetized zone 62 may be thought of as a discrete cylindrical magnet having a north N pole on one longitudinal end thereof and a south S pole on an opposing longitudinal end thereof such that a longitudinal magnetic flux 68 is imparted to the tubular 60. Tubular 60 further includes a single pair of opposing north-north NN poles 65 at the midpoint thereof. The purpose of the opposing magnetic poles 65 is to focus magnetic flux outward from tubular 60 as shown at 70 (or inward for opposing south-south poles as shown at 72).
While the above described method of magnetizing wellbore tubulars has been successfully utilized in well twinning applications, there is room for yet further improvement. For example, it has been found that the above described longitudinal magnetization method can result in a somewhat non-uniform magnetic flux density along the length of a casing string at distances of less than about 6-7 meters. If unaccounted, the non-uniform flux density can result in distance errors on the order of about ±10 percent during well twinning operations. While such distance errors are typically within specification for most well twinning operations, it would be desirable to improve the accuracy of distance calculations between the target and twin wells.
Therefore, there exists a need for an improved apparatus and method for magnetizing wellbore tubulars. In particular, a method of magnetization that results in improved magnetic flux uniformity along the length of a string of magnetized tubulars would be advantageous.